Overlapping facets

ABSTRACT

Specific management of configuration of overlap of facets reduces non-uniformity in an image outcoupled toward a nominal point of observation. A waveguide including at least two parallel surfaces, first, middle, and last partially reflecting facets are configured such that in a geometrical projection of the facets onto one of the surfaces the facets overlap, preferably with adjacent facets overlapping and non-adjacent facets starts and ends coinciding along at least a portion of the waveguide.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of provisional patent application(PPA) Ser. No. 62/474,614 filed 22 Mar. 2017 by the present inventors,which is incorporated by reference in its entirety herein.

FIELD OF THE INVENTION

The present invention generally relates to optics, and in particular, itconcerns uniform reflection.

BACKGROUND OF THE INVENTION

One of the important applications for compact optical elements is withhead-mounted displays (HMD), in which an optical module serves as bothan imaging lens and a combiner, whereby a two-dimensional display isimaged to infinity and reflected into the eye of an observer. Thedisplay can be obtained directly from either a spatial light modulator(SLM) such as a cathode ray tube (CRT), a liquid crystal display (LCD),an organic light emitting diode array (OLED), a scanning source orsimilar devices, or indirectly, by means of a relay lens or an opticalfiber bundle. The display includes an array of elements (pixels) imagedto infinity by a collimating lens and transmitted into the eye of theviewer by means of a reflecting, or partially reflecting, surface actingas a combiner for non-see-through or see-through applications,respectively. Typically, a conventional, free-space optical module isused for this purpose. As the desired field-of-view (FOV) of the systemincreases, such a conventional optical module necessarily becomeslarger, heavier, and bulkier, rendering the device impractical, even formoderate performance. These are major drawbacks for all kinds ofdisplays, but especially so for head-mounted applications wherein thesystems must necessarily be as light and as compact as possible.

The strive for compactness has led to several different complex opticalsolutions, all of which, on one hand, are still not sufficiently compactfor most practical applications, and on the other hand, are difficult tomanufacture. Furthermore, the eye-motion-box (EMB) of the opticalviewing angles resulting from these designs is usually verysmall—typically less than 8 mm. Hence, the performance of the opticalsystems are very sensitive even to small movements relative to the eyeof the viewer, and do not allow sufficient pupil motion for convenientreading of a displayed text.

SUMMARY

According to the teachings of the present embodiment there is providedan optical device including: a waveguide having: a first of at least onepair of surfaces parallel to each other; a first region at which lightis coupled into the waveguide; and a first sequence of facets including:a first facet: located proximally to the first region; and having afirst width in a direction between the first pair of surfaces; a lastfacet: at a distal end of the first sequence of facets from the firstregion; and having a third width in a direction between the first pairof surfaces; and one or more middle facets: between the first facet andthe last facet; and having a second width in a direction between thefirst pair of surfaces; wherein each of the facets width is in a planeof the facet; is an at least partially reflecting surface; is at anoblique angle to the first pair of surfaces; has a facet-start on aproximal side of the facet width; and has a facet-end on a distal sideof the facet width; and wherein a geometrical projection is onto one ofthe first pair of surfaces in a direction of a nominal ray outcoupledfrom the waveguide, the nominal ray being a central ray of the lightbeing coupled out of the waveguide, the geometrical projection of thelast facet and each of the one or more middle facets overlaps arespective the geometrical projection of an adjacent the one or moremiddle facets and the first facet, and the geometrical projection of thefacet-start of the last facet and each of the one or more middle facetscoinciding with a respective the geometrical projection of anon-adjacent facet-end of the one or more middle facets and the firstfacet, the coinciding along at least a portion of the waveguide.

In an optional embodiment, the first width of the first facet is lessthan the second width of the one or more middle facets. In anotheroptional embodiment, a number of the facets is crossed by the nominalray outcoupled from the waveguide, the number of facets being constantfor all of the first sequence of facets. In another optional embodiment,the light corresponds to an image and the central ray is a center rayfrom a center of the image. In another optional embodiment, the lightcorresponds to an image and the central rays corresponds to a centralpixel of the image. In another optional embodiment, the last facet has areflectivity that is substantially 100% of a nominal reflectivity, thenominal reflectivity being the total reflection needed at a specificlocation in the waveguide. In another optional embodiment, the thirdwidth is less than the second width. In another optional embodiment, thethird width is substantially half of the second width. In anotheroptional embodiment, the one or more middle facets is selected from thegroup consisting of: one; two; three; four; five; and a plurality. Inanother optional embodiment, a constant number of facets overlap in aline of sight toward a nominal point of observation of the lightcoupling out of the waveguide via one of the first pair of surfaces. Inanother optional embodiment, a width of one of the facets of the firstsequence of facets varies monotonically relative to a width of anadjacent one of the facets of the first sequence of facets. In anotheroptional embodiment, a spacing between one pair of adjacent facets ofthe first sequence of facets varies monotonically relative to anadjacent spacing between another pair of adjacent facets of the firstsequence of facets. In another optional embodiment, the light from thefirst region is such that at least a portion of the light encounters thefirst facet before encountering one of the one or more middle facets. Inanother optional embodiment, a spacing between adjacent facets is largerthan the coherence length of the light being coupled into the waveguide.

In an optional embodiment, the first width is substantially equal to thesecond width; and the first facet has a first section corresponding tothe geometrical projection of the first facet that is nonoverlappingwith the geometrical projection of an adjacent middle facet. In anotheroptional embodiment, the first section is transparent to the light. Inanother optional embodiment, the first section has a reflectivitysubstantially twice a reflectivity of an adjacent facet. In anotheroptional embodiment, the facets have uniform partial reflectivity acrossthe facet.

In an optional embodiment, the waveguide further has: a second pair ofsurfaces parallel to each other and non-parallel to the first pair ofsurfaces; and the facets configured such that, when an image is coupledinto the waveguide at the first region with an initial direction ofpropagation at a coupling angle oblique to both the first and secondpairs of surfaces, the image advances by four-fold internal reflectionalong the waveguide. In another optional embodiment, the second pair ofsurfaces are perpendicular to the first pair of surfaces. In anotheroptional embodiment, the facets is at an oblique angle to the secondpair of surfaces.

In an optional embodiment, the first width of the first facet issubstantially equal to the second width of the middle facets; a firstreflectivity of the first facet is greater than 50% of a nominalreflectivity; a second facet adjacent to the first facet has a secondreflectivity such that the second reflectivity plus the firstreflectivity are substantially the nominal reflectivity; a third facetadjacent to the second facet has a third reflectivity greater than 50%of the nominal reflectivity and less than the first reflectivity; and afourth facet adjacent to the third facet has a fourth reflectivity suchthat the fourth reflectivity plus the third reflectivity aresubstantially the nominal reflectivity.

In another optional embodiment, the first width of the first facet issubstantially equal to the second width of the middle facets; a sequenceof beginning odd facets includes the first facet, and a given number ofevery other facets from the first facet; a sequence of beginning evenfacets includes a second facet adjacent to the first facet, and a givennumber of every other facets from the second facet; a first set offacets includes a first odd facet from the sequence of beginning oddfacets and a corresponding first even facet from the sequence ofbeginning odd facets; the first odd facet having a first reflectivitygreater than 50% of a nominal reflectivity; the first even facet havinga second reflectivity such that the second reflectivity plus the firstreflectivity are substantially the nominal reflectivity; each subsequentset of facets including a next odd and even facets from respectivesequences of beginning odd and even facets; each of the odd facets fromthe subsequent sets having an odd reflectivity greater than 50% of thenominal reflectivity and less than a reflectivity of an odd facet from aprevious set; and each of the even facets from the subsequent setshaving an even reflectivity such that adding the odd reflectivity to acorresponding even facet's even reflectivity is substantially thenominal reflectivity.

BRIEF DESCRIPTION OF FIGURES

The embodiment is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is a side view of a prior art folding optical device.

FIG. 2 is a side view of an exemplary light-guide optical element.

FIGS. 3A and 3B illustrate the desired reflectance and transmittancecharacteristics of selectively reflecting surfaces, for two ranges ofincident angles.

FIG. 4 is a diagram illustrating an exemplary configuration of alight-guide optical element.

FIG. 5 is a diagram illustrating another configuration of a light-guideoptical element.

FIG. 6 is a diagram illustrating detailed sectional views of atransverse pupil expansion one-dimensional waveguide having asymmetrical structure.

FIG. 7 is a diagram illustrating a method to expand a beam along twoaxes utilizing a double LOE configuration.

FIG. 8 is a diagram illustrating another method to expand a beam alongtwo axes utilizing a double LOE configuration.

FIG. 9 illustrates an exemplary embodiment of LOEs embedded in astandard eyeglasses frame.

FIG. 10A is a schematic view of a waveguide with non-overlapping facets,illustrating the effects of variation on image uniformity.

FIG. 10B is a schematic view of a waveguide with overlapping facets,illustrating the effects of variation on image uniformity.

FIG. 11A to FIG. 11C are exemplary alternative configurations forimplementation of overlapping facets, having different angularpropagation configurations.

FIG. 12A and FIG. 12B are schematic side and front representations,respectively, of a two dimensional optical aperture multiplier.

FIG. 12C and FIG. 12D are schematic diagrams illustrating two possiblegeometries of image rays propagating relative to partially reflectinginternal facets in waveguides from the optical aperture multiplier ofFIG. 12A and FIG. 12B.

FIG. 13 is a schematic isometric view illustrating an implementation ofa 2D waveguide with internal partially reflective facets inclinedobliquely relative to relative to both sets of elongated parallelexternal faces.

FIG. 14A and FIG. 14B are schematic side and front representations,respectively, of an optical aperture multiplier, constructed withoverlapping facets.

FIG. 15A and FIG. 15B are schematic side and front representations,respectively, of an optical aperture multiplier, altering theconstruction of FIG. 14A and FIG. 14B to perform expansion with a freespace optical arrangement.

FIG. 16A to FIG. 16C are sketches of exemplary facet implementations.

FIG. 17A is a rough sketch of double facets.

FIG. 17B is a rough sketch of varying facet spacing.

FIG. 17C is a rough sketch of decreasing facet spacing from the proximalend to distal end of the waveguide.

FIG. 17D is a rough sketch of varying facet width.

FIG. 18 is a rough sketch of applying overlapping facets to asymmetrical structure.

FIG. 19A is a graph of total nominal reflectivity in a doubleoverlapping configuration.

FIG. 19B is an exemplary graph of total nominal reflectivity in a doubleoverlapping configuration using a change in alternating facetreflectivity.

FIG. 20A illustrates a process which may be used to produce a waveguidewith overlapping facets.

FIGS. 20B-20E are an exemplary procedure for attachment of a couplingprism.

FIGS. 21A-21D are further details of an exemplary procedure for creatinga waveguide with overlapping facets.

ABBREVIATIONS AND DEFINITIONS

For convenience of reference, this section contains a brief list ofabbreviations, acronyms, and short definitions used in this document.This section should not be considered limiting. Fuller descriptions canbe found below, and in the applicable Standards.

1D—one-dimensional

2D—two-dimensional

CRT—cathode ray tube

EMB—eye-motion-box

FOV—field-of-view

HMD—head-mounted display

HUD—head-up display

LCD—liquid crystal display

LOE—light-guide optical element

OLED—organic light emitting diode array

SLM—spatial light modulator

TIR—total internal reflection

DETAILED DESCRIPTION

The principles and operation of the system according to a presentembodiment may be better understood with reference to the drawings andthe accompanying description. A present invention is an optical deviceto generate uniform reflection toward an observer.

Specific management of configuration of overlap of facets reducesnon-uniformity in an image outcoupled toward a nominal point ofobservation. A waveguide including at least two surfaces, first, middle,and last partially reflecting facets are configured such that in ageometrical projection of the facets onto one of the surfaces the facetsoverlap, preferably with adjacent facets overlapping and non-adjacentfacets starts and ends coinciding along at least a portion of thewaveguide.

Basic Technology—FIGS. 1 to 9

FIG. 1 illustrates a conventional prior art folding optics arrangement,wherein the substrate 2 is illuminated by a display source 4. Thedisplay is collimated by a collimating optics 6, e.g., a lens. The lightfrom the display source 4 is coupled into substrate 2 by a firstreflecting surface 8, in such a way that the main ray 11 is parallel tothe substrate plane. A second reflecting surface 12 couples the lightout of the substrate and into the eye of a viewer 14. Despite thecompactness of this configuration, this configuration sufferssignificant drawbacks. In particular, only a very limited FOV can beachieved.

Refer now to FIG. 2 is a side view of an exemplary light-guide opticalelement (LOE). To alleviate the above limitations, the presentembodiment utilizes an array of selectively reflecting surfaces,fabricated within a light-guide optical element (LOE). The firstreflecting surface 16 is illuminated by a collimated display light ray(beams) 18 emanating from a light source (not shown) located behind thedevice. For simplicity in the current figures, only one light ray isgenerally depicted, the incoming light ray 38 (also referred to as the“beam” or the “incoming ray”). Other rays of incoming light, such asbeams 18A and 18B may be used to designate edges of the incident pupil,such as a left and right edge of an incoming light pupil. Generally,wherever an image is represented herein by a light beam, it should benoted that the beam is a sample beam of the image, which typically isformed by multiple beams at slightly differing angles each correspondingto a point or pixel of the image. Except where specifically referred toas an extremity of the image, the beams illustrated are typically acentroid of the image.

The reflecting surface 16 reflects the incident light from the sourcesuch that the light is trapped inside a waveguide 20 by total internalreflection. The waveguide 20 is also referred to as a “planar substrate”and a “light-transmitting substrate.” The waveguide 20 includes at leasttwo (major) surfaces parallel to each other, shown in the current figureas a lower (major) surface 26 and an upper (major) surface 26A.

Incoming light ray 38 enters the substrate at a proximal end of thesubstrate (right side of the figure). Light propagates through thewaveguide and one or more facets, normally at least a plurality offacets, and typically several facets, toward a distal end of thewaveguide (left side of the figure). Light propagates through thewaveguide in both an initial direction 28 of propagation, and anotherdirection 30 of propagation.

After several reflections off the surfaces of the substrate 20, thetrapped waves reach an array of selectively reflecting surfaces 22,which couple the light out of the substrate into the eye 24 of a viewer.In alternative configurations, the selectively reflecting surfaces 22are immediately after light ray 18 enters the substrate, without firstreflecting off the surfaces of the substrate 20.

Internal, partially reflecting surfaces, such as selectively reflectingsurfaces 22 are generally referred to in the context of this document as“facets.” In the limit, facets can also be entirely reflecting (100%reflectivity, or a mirror, for example the last facet at the distal endof a substrate), or minimal-reflecting. For augmented realityapplications, the facets are partially reflecting, allowing light fromthe real world to enter via upper surface 26A, traverse the substrateincluding facets, and exit the substrate via lower surface 26 to the eye24 of the viewer. For virtual reality applications, the facets may havealternative reflectivities, such as the first coupling in mirror having100% reflectivity, as the image light from the real world does not haveto traverse this mirror. The internal partially reflecting surfaces 22generally at least partially traverse the waveguide 20 at an obliqueangle (i.e., neither parallel nor perpendicular) to the direction ofelongation of the waveguide 20.

References to reflectivity are generally with respect to the nominalreflectivity. The nominal reflectivity being the total reflection neededat a specific location in the substrate. For example, if thereflectivity of a facet is referred to as 50%, generally this refers to50% of the nominal reflectivity. In a case where the nominalreflectivity is 10%, then 50% reflectivity results in the reflectivityof the facet being 5%. One skilled in the art will understand the use ofpercentages of reflectivity from context of use. Partial reflection canbe implemented by a variety of techniques, including, but not limited totransmission of a percentage of light, or use of polarization.

FIGS. 3A and 3B illustrate a desired reflectance behavior of selectivelyreflecting surfaces. In FIG. 3A, the ray 32 is partially reflected fromfacet 34 and coupled out 38B of the substrate 20. In FIG. 3B, the ray 36is transmitted through the facet 34 without any notable reflection.

FIG. 4 is a detailed sectional view of an array of selectivelyreflective surfaces that couple light into a substrate, and then outinto the eye of a viewer. As can be seen, a ray 38 from the light source4 impinges on the first partially reflective surface. Part of the ray 41continues with the original direction and is coupled out of thesubstrate. The other part of the ray 42 is coupled into the substrate bytotal internal reflection. The trapped ray is gradually coupled out fromthe substrate by the other two partially reflecting surfaces 22 at thepoints 44. The coating characteristics of the first reflecting surface16 should not necessarily be similar to that of the other reflectingsurfaces 22, 46. This coating can be a simpler beam-splitter, eithermetallic, dichroic or hybrid metallic-dichroic. Similarly, in a case ofa non-see-through system, the last reflecting surface 46 can be a simplemirror.

FIG. 5 is a detailed sectional view of an apparatus including an arrayof reflective surfaces wherein the last surface 46 is a total reflectingmirror. It is true that the extreme left part of the last reflectingsurface 46 cannot be optically active in such a case, and the marginalrays 48 cannot be coupled out from the substrate. Hence, the outputaperture of the device will be slightly smaller. However, the opticalefficiency can be much higher and fabrication process of the LOE can bemuch simpler.

It is important to note that, unlike the configuration illustrated inFIG. 2, there is a constraint on the orientation of the reflectivesurfaces 16 and 22. In the former configuration all the light is coupledinside the substrate by the reflective surface 16. Hence, surface 16need not be parallel to surfaces 22. Moreover, the reflecting surfacesmight be oriented such that the light will be coupled out from thesubstrate in the opposite direction to that of the input waves. For theconfiguration illustrated in FIG. 4, however, part of the input light isnot reflected by surface 16, but continues in an original direction ofthe input light 38 and is immediately coupled-out from the substrate asoutput light 41. Hence, to ensure that all the rays originating from thesame plane wave will have the same output direction, it is not enoughthat all the reflecting surfaces 22 are parallel to each other, butsurface 16 should be parallel to these surfaces as well.

Refer again to FIG. 4 describes a system having two reflective surfacesfor coupling the light out of the substrate, however, any number ofreflective surfaces can be used according to the required outputaperture of the optical system and the thickness of the substrate.Naturally, there are cases where only one coupling-out surface isrequired. In that case, the output aperture will essentially be twicethe size of the input aperture of the system. The only requiredreflecting surfaces for the last configuration are simple beam-splittersand mirrors.

In the apparatus described in FIG. 4, the light from the display sourceis coupled into the substrate at the end of the substrate, however,there are systems where it is preferred to have a symmetric system. Thatis, the input light should be coupled into the substrate at the centralpart of the substrate.

FIG. 6 is a diagram illustrating detailed sectional views of atransverse pupil expansion one-dimensional waveguide having asymmetrical structure. The current figure illustrates a method tocombine two identical substrates, to produce a symmetric optical module.As can be seen, part of the light from the display source 4 passesdirectly through the partially reflecting surfaces out of the substrate.The other parts of the light are coupled into the right side of thesubstrate 20R and into the left side of the substrate 20L, by thepartially reflecting surfaces 16R and 16L, respectively. The trappedlight is then gradually coupled out by the reflecting surfaces 22R and22L, respectively. Apparently, the output aperture is three times thesize of the input aperture of the system, the same magnification asdescribed in FIG. 8. However, unlike the system there, the system hereis symmetric about the cemented surface 29 of the right and leftsubstrates.

Refer now to FIG. 7 and FIG. 8, exemplary implementations of FIG. 5 andFIG. 6 on top of a waveguide. The configurations of FIG. 5 and FIG. 6expand the incoming image laterally. The apparatus of FIG. 5 can be usedto implement the first LOE 20 a of FIG. 7, the apparatus of FIG. 6 canbe used to implement the first LOE 20 a′ of FIG. 8, and the apparatus ofFIG. 2 can be used to implement the second LOE 20 b.

FIG. 7 illustrates an alternative method to expand the beam along twoaxes utilizing a double LOE configuration. The input wave 90 is coupledinto the first LOE 20 a, which has an asymmetrical structure similar tothat illustrated in FIG. 5, by the first reflecting surface 16 a andthen propagates along the η axis. The partially reflecting surfaces 22 acouple the light out of first LOE 20 a and then the light is coupledinto the second asymmetrical LOE 20 b by the reflecting surface 16 b.The light then propagates along the axis and is then coupled out by theselectively reflecting surfaces 22 b. As shown, the original beam 90 isexpanded along both axes, where the overall expansion is determined bythe ratio between the lateral dimensions of the elements 16 a and 22 b.The configuration given in FIG. 7 is just an example of a double-LOEsetup. Other configurations in which two or more LOEs are combinedtogether to form complicated optical systems are also possible.

Refer now to FIG. 8, a diagram illustrating another method to expand abeam along two axes utilizing a double LOE configuration. Usually, thearea where the light is coupled into the second LOE 20 b by the surface16 b cannot be transparent to the external light and is not part of thesee-through region. Hence, the first LOE 20 a need not be transparentitself. As a result, it is usually possible to design the first LOE 20 ato have a symmetric structure, as can be seen in the current figure,even for see-through systems. The second LOE 20 b has an asymmetricalstructure that enables the user to see the external scene. In thisconfiguration, part of the input beam 90 continues along the originaldirection 92 into the coupling-in mirror 16 b of the second LOE 20 b,while the other part 94 is coupled into the first LOE 20 a′ by thereflecting surfaces 16 a, propagates along the η axis and is thencoupled into the second LOE 20 b by the selectively reflecting surfaces22 a. Both parts are then coupled into the second asymmetrical LOE 20 bby the reflecting surface 16 b, propagate along the axis, and are thencoupled out by the selectively reflecting surfaces 22 b.

FIG. 9 illustrates an embodiment of LOEs 20 a/20 a′ and 20 b embedded ina standard eyeglasses frame 107. The display source 4, and the foldingand the collimating optics 6 are assembled inside the arm portions 112of the eyeglasses frame, just next to LOE 20 a/20 a′, which is locatedat the edge of the second LOE 20 b. For a case in which the displaysource is an electronic element, such as a small CRT, LCD, or OLED, thedriving electronics 114 for the display source might be assembled insidethe back portion of the arm 112. A power supply and data interface 116is connectable to arm 112 by a lead 118 or other communication meansincluding radio or optical transmission. Alternatively, a battery andminiature data link electronics can be integrated in the eyeglassesframe. The embodiment described in FIG. 9 is only an example. Otherpossible head-mounted displays arrangements can be constructed,including assemblies where the display source is mounted parallel to theLOE plane, or in the upper part of the LOE.

Additional details of this basic technology can be found in U.S. Pat.No. 7,643,214.

First Embodiment—FIGS. 10A to 21D

Refer now to FIG. 10A, a schematic view of a waveguide withnon-overlapping facets, illustrating the effects of variation on imageuniformity. A source of perceived non-uniformity relates to angularoverlap of internal facets in different fields of view. In the region ofwaveguide (10 or 20, see FIG. 12A and FIG. 12B) illustrated here, thewaveguide contains internal facets (two are depicted as last facet 2515and first facet 2517). Most of the out-coupled light is reflected from asingle internal facet. However, at the edge of the facets, there isnon-uniformity at off-axis angles. For a region of the FOV pointing tothe left (marked as solid arrows), a conventional gap area 2520 (alsogenerally referred to as an “underlapping area”, “black line” area, or“dark strip” area) will not reflect any light, since at this angle thereis an effective gap between the light reflected by the last facet 2515and the first facet 2517, resulting in a dark strip in the perceived. Onthe other hand, light out-coupled to the right (marked as dashed arrows)has a conventional bright area 2525 (also generally referred to as a“partially overlapping” area, or “intense” area) within which there isoverlap of the light reflected from 2515 and 2517 so that the waveguidewill reflect almost twice the amount of light. Therefore, thenon-uniformity in FIG. 10A will vary between roughly 200% and 0% of themedian image intensity across the extended aperture in different regionsof the FOV and eye positions.

Refer now to FIG. 10B, a schematic view of a waveguide with overlappingfacets, illustrating the effects of variation on image uniformity.Substantial overlap is introduced between the facets, as illustrated inthe current figure. In this case, the spacing between adjacent facets ishalved, resulting in most parts of the FOV at most eye positionsreceiving illumination from the image via overlaid reflections from twofacets. In this exemplary case, a single middle facet 2535 is configuredbetween the last facet 2515 and the first facet 2517. Near the angularextremities of the image and the extremities of the facets, there willstill be changes in the number of overlapping facets which contribute tocertain regions of the image, as illustrated by underlapping area 2540which originates from only one facet (the middle facet 2535) and brightarea 2545 which is contributed to by three adjacent facets (2517, 2535,and 2515. Therefore, the output non-uniformity will vary between 50% and150% of the median reflectivity.

The light from the first half (light propagating from the right) offacet 2517 will couple out as reduced energy (ray/output beam 2546)since at this position there is no overlapping of the next facet 2535i.e. there is only one facet reflect the light to the observer. The samereduced power happens at the last half of facet 2515 (ray/output beam2547). In these regions, the reflectivity will be 50% of the medianreflectivity.

A feature of the current embodiment is management of configuration ofoverlapping of the facets, specifically optimizing the overlap to obtaina constant number of facets (more than one) reflecting light onto theobserver. In other words, at least two facets reflect light toward a FOVof an observer.

Refer now to FIG. 11A to FIG. 11C, exemplary alternative configurationsfor implementation of overlapping facets for different reflection(angular propagation) configurations (these configurations are alsodescribed in FIG. 12C and FIG. 12D). For simplicity in the currentfigures, only one light ray is depicted, the incoming light ray 38 (alsoreferred to as the “beam”), with corresponding out-coupling rays 38B.For simplicity, no cross coupling is depicted. Some of the outcouplingrays 38B1 pass through internal facets 2560 and some outcoupling rays38B2 couple directly out.

In configuration FIG. 11A, the incoming ray 38 crosses the internalfacets 2560 from both sides of the internal facets 2560. A firstcrossing is from behind facet 2562, and in this crossing a coating onthis side of the facet should be transparent for this shallow angle. Thebeam also crosses facet 2564 from another side, that is on a front side,opposite the behind side, and at this exemplary steeper angle thecoating of the facet should be partial reflective so part of the lightis directed out of the waveguide. (A similar single facet is describedin U.S. Pat. No. 7,391,573 B2).

In configurations shown in FIG. 11B and FIG. 11C, the angle of theinternal facets 2560 and the direction of light propagation is set sothat the beam (incoming ray 38) passes the internal facets 2560 alwaysfrom the same side of the facets. The coating in the facets can be usedto set the reflectivity and transmissivity so the appropriate beam (38B)is reflected out.

In FIG. 11B the beam 38 crosses the initial internal facets 2560 closeto perpendicular, as shown at point 2568 where the facet coating isdesigned to be transparent. A second crossing is at a shallow angle asshown at point 2570 (further from perpendicular) where the coating isdesigned to be partial reflector so part of the light is coupled out(38B).

In FIG. 11C, the facet coating is set to be partial reflector close toperpendicular as shown at point 2574 and at an angle further fromperpendicular to be transparent as shown at point 2576.

Referring now to the drawings, FIGS. 12A-12D illustrate overlappingfacets in one-dimensional (1D) and two-dimensional (2D) waveguides of anoptical aperture multiplier. In general, terms, an optical aperturemultiplier according to an embodiment of the present invention includesa first optical waveguide 10 having a direction of elongationillustrated arbitrarily herein as corresponding to the “x-axis”. Firstoptical waveguide 10 has first and second pairs of parallel faces 12 a,12 b, 14 a, 14 b forming a rectangular cross-section. A plurality ofinternal partially reflecting surfaces 40 at least partially traversefirst optical waveguide 10 at an oblique angle (i.e., neither parallelnor perpendicular) to the direction of elongation.

The optical aperture multiplier preferably also includes a secondoptical waveguide 20, optically coupled with first optical waveguide 10,having a third pair of parallel faces 22 a, 22 b forming a slab-typewaveguide, i.e., where the other two dimensions of waveguide 20 are atleast an order of magnitude greater than the distance between third pairof parallel faces 22 a, 22 b. Here too, a plurality of partiallyreflecting surfaces 45 preferably at least partially traverse secondoptical waveguide 20 at an oblique angle to the third pair of parallelfaces.

The optical coupling between the waveguides, and the deployment andconfiguration of partially reflecting surfaces 40, 45 are such that,when an image is coupled into first optical waveguide 10 with an initialdirection 28 of propagation (for example, light ray 38) at a couplingangle oblique to both the first and second pairs of parallel faces 12 a,12 b, 14 a, 14 b, the image advances by four-fold internal reflection(images a1, a2, a3 and a4) along first optical waveguide 10, with aproportion of intensity of the image reflected at partially reflectingsurfaces 40 so as to be coupled into second optical waveguide 20, andthen propagates through two-fold reflection (images b1, b2) withinsecond optical waveguide 20, with a proportion of intensity of the imagereflected at partially reflecting surfaces 45 so as to be directedoutwards from one of the parallel faces as a visible image c, seen bythe eye 47 of a user.

Turning now more specifically to FIG. 12A and FIG. 12B, schematic sideand front representations, respectively, of a two-dimensional opticalaperture multiplier, showing a first illustration of an implementationof the above description. First waveguide 10 is referred to herein as atwo-dimensional (2D) waveguide in the sense that first waveguide 10guides the injected image in two dimensions by reflection between twosets of parallel faces (in this case the first and second pairs ofparallel faces 12 a, 12 b, 14 a, 14 b), while second waveguide 20 isreferred to as a one-dimensional (1D) waveguide, guiding the injectedimage in only one dimension between one pair of parallel faces (in thiscase the third pair of parallel faces 22 a, 22 b).

A further improvement to reducing non-uniformity may result from theintroduction of “multipath” images that are generated by the overlappinginternal facets, as depicted in FIG. 12B. A similar process exists ingeneral in overlapping facet implementations. The light propagatingwithin 2D waveguide 10 (marked as solid arrows and designated “a”) iscoupled out (designated “b”), but some of the light from b isback-coupled to a′ (marked as dashed arrows) before being coupled out asb′ (marked as dashed arrows). This back-and-forth coupling between ‘a’and ‘b’ causes averaging of the intensity across the aperture whilemaintaining light parallelism, thereby further improving lightuniformity. This improvement can also be implemented in other waveguideswith a similar process using overlapping facets, such as shown in FIG.12A for 1D waveguide 20. The light propagating within 1D waveguide 20(marked as solid arrows and shown as beams “b1” and “b2”) is coupled out(shown as beam “c”), but some of the light from beam c is back-coupledto b2′ (marked as dashed arrows) before being coupled out as beams c3and c4 (marked as dashed arrows).

Light beam 38 from an optical image generator (not depicted) is injectedinto first waveguide 10 at an angle. Consequently, the light propagatesalong waveguide 10 while being reflected from all four external faces ofthe waveguide as shown in the side view of FIG. 12A. In this process,four conjugate beam vectors are generated a1, a2, a3, and a4 thatrepresent the same image as the image is reflected internally by thefaces.

The angle of beam 38 that is injected into waveguide 10 is set toreflect from all four external faces of this waveguide. The light beamshould reflect from the bottom face 12 b of first waveguide 10, i.e.,the face adjacent to second waveguide 20, at shallow (grazing) anglesand should preferably transmit from 10 into 20 at steep angles. Thisproperty can be achieved by total internal reflection (TIR) or byoptical coating. A diffractive pattern can also perform this opticalproperty by combining diffraction with transmission on the same surface.Reflection from the other three faces 12 a, 14 a, and 14 b of firstwaveguide 10 can be generated the same way or by use of a reflectingcoating.

Part of the guided light-beams (for example beam a1 and beam a2) withinfirst waveguide 10 are reflected by the internal parallel partialreflectors (facets) 40 downward onto an input coupling surface of secondwaveguide 20. In second waveguide 20, these beams are defined asexemplary beams b1 and b2. In this process, the overlappingconfiguration causes cross-coupling, thereby improving uniformitywithout degradation of image quality (as described).

Beams b1 and b2 are reflected by the external faces and becomeconjugate, i.e., beam b1 is reflected to be beam b2 and vice versa (asdepicted in FIG. 12A). The external front and back faces 14 a, 14 b offirst waveguide 10 should be parallel to each other and, in thisimplementation, to the corresponding external faces 22 a, 22 b of secondwaveguide 20. Any deviation from parallelism will cause the coupledimages from beams b1 and b2 not to be precise conjugate images, andimage quality will degrade.

The internal facets 45 within second waveguide 20 reflect beam b2outside the waveguides and into the eye of the observer 47. The internalfacets 45 can also be overlapping, thereby further improving imageuniformity as described for facets 40.

The reflection process by the internal facets in waveguides 10 and 20 isfurther explained in FIG. 12C and FIG. 12D. Two basic configurations aredepicted, and differ by the relative angles of the light beams and thefacets. In this schematic illustration, the beams a1, a2 and b1 aredepicted as same vector (reference will be only to beam b1) since thesame geometrical considerations apply to each as observed from a sideview of the corresponding waveguide. Beams a3, a4 and b2 are alsodepicted as same vector (reference will be only to b2).

Light beams b2 are actually a bundle of rays propagating in samedirection as depicted by two vectors in FIG. 12C. In this case, onevector is reflected by the external face to become beam b1 and onto theinternal facet 40 (or 45) where part of the one vector is reflected asbeam c1. The other beam b2 vector is reflected directly by facet asvector beam c2. The vector beams c1 and c2 represent the normal imageand ghost image not necessarily in this order. In this configuration,beams b1 and b2 impinge on facet 45 from the same side.

FIG. 12D describes essentially the same process but where the geometryis such that beams b1 and b2 impinge on facet 40 (or 45) from oppositesides.

In both cases, the magnitude of reflection for images c1 and c2 in S andP polarizations is determined by the coating on these facets.Preferably, one reflection is the image and the other is suppressedsince the other image corresponds to an unwanted “ghost” image. Suitablecoatings for controlling which ranges of incident beam angles arereflected and which ranges of incident beam angles are transmitted areknown in the art, and can be found described in detail in U.S. Pat. Nos.7,391,573 and 7,457,040, coassigned with the present invention.

FIG. 13 illustrates an alternative implementation in which the partiallyreflecting surfaces of first waveguide 10, here designated 155, are atan oblique angle to both faces 12 a and 14 a. (The dashed lines areintended to facilitate visualizing the inclination of the facets, byshowing a plane perpendicular to both external faces, and anotherinclined relative to only one face.)

Refer now to FIG. 14A and FIG. 14B are schematic side and frontrepresentations, respectively, of an optical aperture multiplier,constructed with overlapping facets. The general operation of thecurrent figures is described above in reference to FIG. 12A and FIG.12B. The overlapping of facets is applied in the 2D waveguide 10 as wellas in the 1D waveguide 20. In this example, in FIG. 14B, the 2Dwaveguide 10 expands the optical aperture latterly (in the currentfigure from right to left) and the 1D waveguide 20 expands the opticalaperture vertically (in the current figure from top to bottom) beforetransmitting the light to the eye of the observer 47.

In FIG. 14A, light (shown as incoming ray 38) is coupled into 2Dwaveguide 10. This waveguide includes overlapping facets 40. Dashedlines are used in the current figure to show alignment of the facets 40,which are shown as double-lines. In this implementation, the first facet40 a and the last facet 40 b have smaller area than the middle facets ofthe internal facets 40. This enables light coupled out (‘b’) of the 2Dwaveguide 10 to be substantially uniform since the outcoupled light ‘b’was originated by a constant number of facets including at the start andend of the 2D waveguide 10. For example, output ray b10 and output rayb20 (actually overlapping when output from waveguide 10, but shownslightly separated in the figure for clarity) produce a combined outputthat was originated by two facets (first facet 40 a and an adjacentfacet of the internal facets 40). Similarly, output rays b30 and b40produce an output from two facets.

For comparison, refer back to FIG. 10B where the light output beam 2546from the first full facet 2517 and the light output beam 2547 from thelast full facet are coupled out as reduced energy. Using the partialfirst and last facets (40 a, 40 b) this reduced energy will be avoidedsince partial first facet 40 a and partial last facet 40 b are shorterto overlap the adjacent facets 40. Note, if the last facet illuminatedis designed to have 100% reflectivity (100% of nominal reflectivity whenused for augmented viewing), then the last facet will perform similar tocomplete facet 2515.

The overlapping facet configuration described for the 2D waveguide 10works similarly for the 1D waveguide 20. Internal facets 45 reflect thelight to the observer 47. The 1D waveguide internal facets 45 areoverlapping as described for 2D waveguide internal facets 40. Similar topartial first and last facets 40A and 40 b, first and last facets 45 aand 45 b have reduced area in order to maintain illumination uniformityas described for 2D waveguide 10.

Refer now to FIG. 15A and FIG. 15B where the basic structure of FIG. 14Aand FIG. 14B is altered replacing the 2D waveguide 10 to perform thelateral expansion with a free space optical arrangement 11 (for exampleas described in FIG. 5 and FIG. 6). The innovative overlapping structureof 1D waveguide 20 is still used to perform the vertical expansion.

Refer now to FIG. 16A to FIG. 16C, sketches of exemplary facetimplementations. The facets can be arranged in a variety of overlappingconfigurations, including but not limited to the amount of overlap,angle of facets with respect to the parallel surfaces (major edges, suchas the pair of lower surface 26 and upper surface 26 a) of the waveguidesubstrate, and reflectivities. Overlapping of facets can be implementedin what are referred to in the context of this document as single(non-overlapping), double, and triple (overlapping) facets. In general,overlap (by definition starting with “double facets”) of two or morefacets is referred to as “multiple facets”, or “multiple overlap”.Additional overlaps beyond triple are possible, as well as partialoverlapping, as will be apparent from the current description andnon-limiting examples. For clarity in the current figures, thepropagation from incoming light ray 38 to outcoupling ray 38B is notshown.

FIG. 16A, for reference, shows a conventional implementation of singlefacets, or no overlap, as described above with reference to FIG. 2.Waveguide 20 includes facets 22, which are shown as double-lines,between the first two surfaces (26, 26A). A first region 54 is an areaat which light (shown as ray 38) is coupled into the substrate. Thesolid arrow shows outcoupling rays 38B crossing only one facet (singlefacet crossing). Note that references to “crossing” facets and thenumber of facets being “crossed” includes counting the facet that is theorigin of the outcoupled ray. Dashed lines are used to show alignment ofthe facets 22. In this single facet configuration, the facets 22 do notoverlap, and specifically are configured with the end of one facetaligned with the beginning of an adjacent facet.

References to alignment will be obvious to one skilled in the art asrelative to a geometrical projection of the facet onto one of thesurfaces. For example, exemplary facet F1 facet-start has a geometricalprojection onto lower surface 26 at point P1. Exemplary facet F2facet-end has a geometrical projection onto lower surface 26 also atpoint P1. Exemplary facet F2 facet-start has a geometrical projectiononto lower surface 26 at point P2. Exemplary facet F3 facet-end has ageometrical projection onto lower surface 26 also at point P2.

FIG. 16B is a sketch of a double facets (double facet crossing, doubleoverlap). This is a preferred implementation, which experiments haveshown to provide good results while minimizing increases inmanufacturing complexity (as compared to higher-level crossings). Thenon-limiting example of a double facet overlap is generally used in thisdescription. A waveguide (light transmitting substrate, waveguide 20)includes overlapping internal facets 40, which are shown asdouble-lines, between the first (two) surfaces (26, 26A). A solid arrowshows incoming light ray 38. Another solid arrow shows a nominal raycrossing two facets and then outcoupled from the substrate (arrowoutcoupling ray 38B). This crossing of two facets (facet F11 and facetF12) is a double facet crossing. As in similar figures, dashed lines areused to show alignment of the facets 40. In this example, a single firstpartial facet 40 a and single last partial facet 40 b are shown.

The waveguide includes at least one pair of surfaces parallel to eachother (lower surface 26 and upper surface 26A, referred to as “firstsurfaces”). A substrate width 52 is a distance between the firstsurfaces. A first region 54 is an area at which light (shown as ray 38)is coupled into the substrate.

The waveguide includes a sequence of facets 56. The sequence of facets56 includes a first facet (40 a), a last facet (40 b), and one or moremiddle facets (40 c). The first facet 40 a is located proximally to thefirst region 54, where proximally is the nearest part of the sequence offacets 56. The first facet has a first width (52 a) in a directionbetween the first surfaces (26, 26 a).

The last facet 40 b is at a distal end 55 of the sequence of facets 56from the first region 54. The last facet 40 b has a third width 52 b ina direction between the first surfaces (26, 26 a).

One or more middle facets 40 c are located between the first facet 40 aand the last facet 40 b. The middle facets (each of) have a second width52 c in a direction between the first surfaces (26, 26 a). For clarity,only one second width 52 c is shown. In a typical implementation, thewidths of all of the middle facets will be equal. However, thisimplementation is not limiting, and the widths of each facet can varyfrom one another, as is described below. The number of middle facets canvary depending on the application. Typical numbers of the one or moremiddle facets include one, two, three, four, five, and a plurality.

Each facet of the sequence of facets 56 is typically an at leastpartially reflecting surface, is at an oblique angle to the surfaces(26, 26 a), has a facet-start on a proximal side of the facet width, andhas a facet-end on a distal side of the facet width. Exemplaryfacet-starts are shown for the first facet 40 a as point 57 a; for amiddle facet adjacent to the first facet 40 a as point 57 m, for a nextmiddle facet as point 57 n, and for the last facet 40 b as point 57 b.Similarly, exemplary facet-ends are shown for the first facet 40 a aspoint 58 a; for a middle facet adjacent to the first facet 40 a as point58 m, for a next middle facet as point 58 n, and for the last facet 40 bas point 58 b.

An alignment of the overlapping of the facets is now described. Tobegin, we define a geometrical projection being onto one of the surfaces(in this case we will use lower surface 26) in a direction of a nominalray 38B outcoupled from the substrate 20. The nominal ray 38B istypically substantially a central ray of the light being coupled out ofthe substrate 20. Generally, the nominal ray 38B is a ray that adesigner wishes to have optimal performance in the ray field. A nominalray 38B can also be the optimal ray for a specific location on thesubstrate 20. In certain particularly preferred embodiments, the nominalray is designed to be perpendicular to the parallel surfaces of thelight guiding optical element, but depending on various designconsiderations, the nominal ray may be inclined relative to a normal tothose parallel surfaces in one or two dimensions. Note that if a nominalray 38B is not perpendicular to the parallel surface (for example 26) ofthe substrate 20, then the nominal ray 38B is at an angle to thesurface, the nominal ray 38B will refract when outcoupling from thesubstrate 20, and be at a different angle outside the substrate 20. Inthe context of this document, normally reference is to the nominal ray38B inside the substrate 20. Usually the nominal ray corresponds to aray from the center or near the center of the incoming image. In someimplementations, the nominal ray is the chief ray of the incoming image.Typically, the incoming light 38 corresponds to an image, and thecentral ray is a center ray from a center of the image. Additionally oralternatively, the incoming light 38 corresponds to an image, and thecentral ray corresponds to a central pixel of the image.

Next, the geometrical projection of the last facet 40 b and each of theone or more middle facets 40 c overlaps a respective geometricalprojection of an adjacent one or more middle facets 40 c and the firstfacet 40 a. In other words, adjacent facets overlap. For example, lastfacet 40 b at the distal end overlaps adjacent left-most (in the figure)middle facet, each of the middle facets 40 c overlaps an adjacent middlefacet, and the right-most middle facet at the proximal end overlaps thefirst facet 40 a.

In addition, the geometrical projection of the facet-start of the lastfacet 57 b and each of the one or more middle facets (such as 57 n, 57m) preferably substantially coincides with a respective geometricalprojection of a non-adjacent facet-end of the one or more middle facets(such as 58 n, 58 m) and the first facet 58 a. In other words, eachfacet-start aligns, or is preferably in close alignment, with anon-adjacent facet-end in the direction of nominal ray outcoupling,(with the obvious exception of the first facet 40 a, as there are nofacet ends with which to align). The coinciding is along at least aportion of the substrate.

Alternatively, the overlapping of facets can be described as a constantnumber of facets overlapping in a line of sight toward a nominal pointof observation of the light coupling out of the substrate via one of thesurfaces. In other words, the nominal point is a typical location of aneye 47 of a user, the most probable location of a pupil of an eye of anobserver. In some applications, the nominal point is the center of theeyeball of the observer. Internal facets are optimized to generateuniform reflection toward the observer by having constant number offacets overlapping in the line of sight toward a nominal point ofobservation.

A feature of the current embodiment is specific management of theconfiguration of the overlap of facets. In this case, of double-facetcrossing, the facet-end of every first facet and middle facet is in thesame line as the center of an adjacent middle facet or last facet.Similarly, the facet-start of every last facet and middle facet is inthe same line as the center of an adjacent middle facet or last facet.In this case, the following exemplary facets have geometricalprojections onto lower surface 26 at the following points:

-   -   Facet-end of facet F11 at point P11;    -   The middle of facet F11 at point P12;    -   Facet-start of facet F11 at point P14;    -   Facet-end of facet F12 at point P12;    -   The middle of facet F12 at point P14;    -   Facet-end of facet F13 at point P14; and    -   Ray 38B crosses facets F11 and F12 at point P13.

Thus, the proximal end of middle facet F11 overlaps the distal end ofadjacent middle facet F12 and the facet-start of middle facet F11 alignswith the facet-end of nonadjacent middle facet F13.

The facets are normally parallel to each other and in constant spacing,that is, a spacing between one pair of adjacent facets of the sequenceof facets is the same as a spacing between another pair of adjacentfacets of the sequence of facets. For example, spacing 59 a betweenfacet F11 and facet F12 is substantially the same as spacing 59 bbetween facet F12 and facet F13. The spacing between adjacent facets istypically larger than the coherence length of the light being coupledinto the substrate. The coherence length is the propagation distanceover which a coherent wave (e.g. an electromagnetic wave) maintains aspecified degree of coherence. Generally, the coherence length is thewavelength squared, divided by spectral width. If facet spacing ischanged along the waveguide, the conditions of overlapping should bepreserved.

In a preferred embodiment, the first width of the first facet is lessthan the second width of the one or more middle facets. In other words,the first facet is a partial facet. In an exemplary implementation, thefirst width is substantially half of the second width.

In another option, the third width is less than the second width. Inother words, the last facet is a partial facet, preferably half thewidth of the middle facets (the third width is substantially half of thesecond width). In another option, the last half facet has a reflectivitythat is substantially 100% of a nominal reflectivity. For example, ifthe nominal reflectivity is 50% (as is the case with double overlap),then at the distal half end of the waveguide the last facet would have areflectivity of 50%. For example in FIG. 10B if half of facet 2517 has100% nominal reflectivity then ray 2546 will have same intensity as restof the outcoupled light. Similarly, if half of facet 2515 has 100%reflectivity, then ray 2547 will also have the same intensity as rest ofthe outcoupled light.

The propagation of the light from the first region is such that at leasta portion of the light encounters the first facet before encounteringone of the one or more middle facets.

Refer now to FIG. 16C is a sketch of triple facets (triple facetcrossing, triple overlap). Similar to the other examples, a waveguide(waveguide 20) includes overlapping internal facets 40, which are shownas double-lines, between the first surfaces (26, 26A). A solid arrowshows a nominal ray crossing three facets and then outcoupled from thesubstrate (arrow outcoupling ray 38B). As in similar figures, dashedlines are used to show alignment of the facets 40. In this example,multiple (specifically two) first partial facets and multiple (two) lastpartial facets are shown.

In general, a number of facets is crossed by the nominal ray outcoupledfrom the waveguide substrate. In the example of a double facet crossing,the number of facets crossed is two. Similarly, in the example of atriple facet crossing the number of facets crossed is three. In general,the number of facets crossed is constant for all of the sequence offacets. Constructing a waveguide with a constant number of facetscrossed can be implemented in a variety of configurations. For exampleas described in reference to FIG. 16B the first width 52 a of the firstfacet 40 a can be substantially half of the second width 52 c of theadjacent middle facet (one facet of the middle facets 40 c). In anotherexample, ¼ of the first facet and ¾ of the adjacent facet can be used.In another example, refer to FIG. 16C where both the first facet 40 aand a first adjacent facet F14 are portions of the width of the nextadjacent facet F15.

Based on the current description using an exemplary 1D waveguide (forexample, the 1D waveguide 20) for implementation of overlapping facets,one skilled in the art will be able to implement overlapping facets fora 2D waveguide (for example, in the 2D waveguide 10) and other waveguideconfigurations.

Refer back to FIG. 14A, FIG. 14B, and FIG. 13. In general, in a 2Dwaveguide, the waveguide includes the first surfaces (26, 26 a, or 12 b,12 a) and a second pair of surfaces (14 a, 14 b). The second surfaces(14 a, 14 b) are parallel to each other and non-parallel to the firstsurfaces (12 b, 12 a). Similar to the widths of facets with respect tothe first surfaces, the first facet has a fourth width in a directionbetween the second surfaces, the last facet has a sixth width in adirection between the second surfaces, and the one or more middle facetshave a fifth width in a direction between the second surfaces. A featureof the 2D waveguide is that the facets are configured such that, when animage is coupled into the waveguide at the first region with an initialdirection of propagation at a coupling angle oblique to both the firstand second surfaces, the image advances by four-fold internal reflectionalong the waveguide,

In an alternative embodiment, the second surfaces are perpendicular tothe first surfaces. In another alternative embodiment, each of thefacets is at an oblique angle to the second surfaces.

Refer now to FIGS. 17A to 17D, rough sketches of alternative facetconfigurations. In the current figures, the facets are parallel.

Refer now to FIG. 17A is a rough sketch of double facets, as describedin detail in FIG. 16B, for comparison. Since the waveguide projects theimage toward the eye 47 of a user, different light rays propagate atdifferent angles, thereby generating overlap and underlap that reduceuniformity (introduce non-uniformity), as was described above withreference to FIG. 10A. The overlapping configuration of FIG. 17A(similarly FIG. 16B, and FIG. 10B) reduces this non-uniformity effectrelative to the non-overlapping configuration of FIG. 10A (similarlyFIG. 16A). For many applications, this double-facet configuration issufficient, and the overlapping sufficiently suppresses non-uniformity.

Refer now to FIG. 17B, a rough sketch of varying facet spacing. Furtherreduction in non-uniformity from the double-facet configuration of FIG.17A is shown in FIG. 17B where a spacing between one pair of adjacentfacets of the sequence of facets varies relative to an adjacent spacingbetween another pair of adjacent facets of the sequence of facets. In apreferred embodiment, the spacing variation is monotonic between onepair of adjacent facets and an adjacent pair of adjacent facets. Forexample, spacing 59 d is greater than spacing 59 c, and spacing 59 e isgreater than spacing 59 d. Note, the variations of spacing of the facetscan be reduced due to refraction of the output rays 38B, as the outputrays 38B bend toward normal when exiting the substrate (not pictured forsimplicity). In the current configuration, different angles of thenominal wave are handled, and overlapping is constant for the observer(eye 47 of a user). In this non-limiting example, the nominal outputrays will always go through two facets. Note, in the current figure thenominal ray 38B at the center of the substrate 20 differs in angle fromthe nominal rays at the ends (such as proximal and distal) of thesubstrate 20.

Refer now to FIG. 17C, a rough sketch of decreasing facet spacing fromthe proximal end to distal end of the waveguide. The waveguide 20includes a first spacing in a first portion of the waveguide and atleast a second spacing in a second portion of the waveguide 20. In thisnon-limiting example, a first portion 61 a includes non-overlappingfacets. A second portion 61 c includes double-overlapping facets, andanother portion 61 e includes triple-overlapping facets. Portions 61 band 61 d are transition portions, or areas of transition from onediscrete overlapping to another discrete overlapping. In alternativeembodiments, the overlapping of the portions can be non-discrete,continually varying, or another spacing configuration designed to managethe effective output intensity from the waveguide.

In order to maintain constant reflected intensity along the waveguide,every facet must have higher reflective coefficient starting from theproximal end and increasing in reflectivity in the direction of thedistal end. This management of reflected intensity improves theuniformity (uniformity of intensity) of the output to the observer. Inthe current figure, the reflectivity of every facet can be maintainedconstant while the spacing between the facets varies according torequired reflectivity. The light is injected into the waveguide from theproximal end (right side of the current figure) and therefore hashighest intensity on the proximal end. On the proximal end, the spacingbetween the facets is the largest, and there is minimal overlappingbetween the facets. As the light propagates along the waveguide (notdepicted) the power of the light is reduced, and higher overlapping ofthe facets compensates for this reduction in power. Thus an overallpower output is maintained along the waveguide.

Continuity can be maintained along the waveguide by non-continuousvariation of an overlapping integer number or by continuous change(non-integer) at narrow spacing of facets, where overlappingdiscontinuity is not observed.

If spacing and height of facets is to be maintained constant across thewaveguide, then an optimization procedure should consider the impact ofoverlap versus underlap of the facets. Overlapping facets offer moreoutput power and more mixing of non-uniformity. Furthermore, overlapcauses intensity change from 100% to 150% (or 100%±20%) while underlapgenerates 50% to 100% (or 100%±33%). In overlap, the relative intensitychange is lower. Thus, the reflectivity of one or more of the facetsvaries from another reflectivity of another one or more facets in thesequence of facets.

Also note that an observer's eye does not respond linearly to intensityvariations, rather the eye has a logarithmic response. This also impliesthat the underlap has more impact on observer perception. Given theabove, more consideration should be given to reducing the underlap atthe cost of increasing overlap.

Refer now to FIG. 17D, a rough sketch of varying facet width. Furtherreduction in non-uniformity from the double-facet configuration of FIG.17A is shown in the current figure, where a width of one of the facetsof the sequence of facets varies relative to a width of an adjacent oneof the facets of the sequence of facets. In a preferred embodiment, thewidth variation is monotonic between one of the facets and an adjacentfacet for the entire sequence of facets. In the current figure, thewidth is shortened from the bottom toward the top of the waveguide asthe sequence of facets is traversed from the proximal end to the distalend. An alternative implementation is to shorten the facet width fromboth sides (from the top and bottom toward the center of each facet). Inthe current example, width 59 e is greater than width 59 d, and width 59f is greater than width 59 e.

In both the implementation of FIG. 17B changing the facet spacing and ofFIG. 17D changing the facet width, the observer (eye 47 of a user) atthe nominal convergence of the light-beams (output rays 38B) will notsee overlap or underlap. However, any variation in eye position willgenerate some overlap/underlap that will be suppressed by the doublefacet configuration.

Refer now to FIG. 18, a rough sketch of applying overlapping facets to asymmetrical structure, such as described in reference to FIG. 6 and FIG.8. In the current figure, only the top transverse waveguide is depicted,similar to the overlapping configuration of FIG. 17B. In thissymmetrical configuration, the first region (similar to FIG. 16B firstregion 54) is an area at which light (shown as ray 38) is coupled intothe substrate, in this case, a region in the middle of the waveguide.Each of the symmetric left and right sides of the waveguide has an areaat which light is coupled into the respective sides of the substrate(alternatively referred to as a first region and second region that areadjacent), with the facets of the left and right sides being equal andof opposite inclination. This symmetrical structure can also beimplemented with the parallel configuration of facets in FIG. 17A andthe varying width configuration of FIG. 17D.

Refer again to FIG. 10B where in an overlapping configuration the (firstfull) facet 2517 and (last full) facet 2515 are partly not overlapped.As described above, specifically the beginning of facet 2517 does notoverlap facet 2535 and the end of facet 2515 does not overlap facet2535. Therefore, the intensity of the light coupled out (2546, 2547) isless intense in these non-overlapped sections. For example, in a doubleoverlap configuration, half the first full facet is not overlapped andhalf the power will be coupled out from the non-overlapped part.

Several techniques can be used to overcome the problem of less intensityat the non-overlapped beginning and end sections.

1. Using shorter facets at start and end, as described above inreference to FIG. 14B elements 40 a and 40 b, and FIG. 16B.

2. Coating the non-overlapped section with a high reflective coatingthat increases the reflectivity of the non-overlapped section relativeto the nominal reflectivity of the other (middle) facets.

3). Gradually changing the characteristic reflectivity of the facetsfrom non-overlapping to overlapping, as described below.

The technique of gradually changing the characteristic reflectivity isnow described using a double overlapping configuration for simplicity,but this technique can be applied for higher overlapping configurations.

Refer now to FIG. 19A, a graph of total nominal reflectivity in a doubleoverlapping configuration. The x-axis shows the facets, starting at afacet numbered “1” (one) that is the first facet proximal to the firstregion 54 at which light is coupled into the waveguide (substrate).Increasing numbered facets are facets subsequent to facet “1” toward thedistal end 55 of the waveguide. The y-axis shows reflectivity as apercent of total nominal reflectivity. Thin-line black boxes are thereflectivity (percentage of nominal reflectivity) of each individualfacet, and the thick black line is the characteristic reflectivity—theeffective reflectivity experienced by an outcoupled ray. Each facet isshown having a constant nominal reflectivity, for example, 50% of thenominal required reflectivity.

The characteristic reflectivity is the sum of the individualreflectivities for a portion of the waveguide at which a ray isoutcoupled. As can be seen, the characteristic reflectivity outcouplingfrom the non-overlapped section from facet “1” in the current example is50% (of nominal) as can be seen in FIG. 10B ray 2546 (or ray 2547). Thecharacteristic reflectivity outcoupling from the overlap of facet “1”and facet “2” (overlapping of two adjacent facets) achieves 100% (ofnominal). Hence there is a discontinuity between the beginning andsubsequent portions of the waveguide (50% versus 100%).

Refer now to FIG. 19B, an exemplary graph of total nominal reflectivityin a double overlapping configuration using a change in alternatingfacet reflectivity. In the current figure, facet “1” (the first facet)is designed to approximate no overlapping (i.e. have approximately 100%the nominal), and facet “2” (the second facet) is designed to haveminimal reflection. Thus, when combined, the first and second facetshave approximately a no-overlapping characteristic reflectivity.Similarly, facet “3” (third facet) has almost as much reflectivity asfacet “1”, but reduced and facet “4” has almost as much reflectivity asfacet “2”, but increased. In the current figure, facets “7” and “8” have50% (nominal) reflectivity resulting in a characteristic reflectivity asin the double overlapping configuration described in FIG. 19A. The dotdashed line represents the reflectivity of odd numbered facets (startingwith more non-overlapping coating parameters) while the dashed linerepresents the reflectivity of even numbered facets (represent theincreasing overlapping property). The thick-black (solid) linerepresents the characteristic reflectivity summing the reflectivity oftwo adjacent facets (as caused by double overlap and the first facetwith no overlap) showing no half-reflectivity of the first half facet,as compared to the 50% characteristic reflectivity of facet “1” of FIG.19A.

While the configuration of FIG. 19A has a discontinuity between thebeginning and subsequent portions of the waveguide (50% versus 100%),the innovative configuration of FIG. 19B reduces this discontinuity. Theresidual discontinuity depends on convergence rate, for example forconvergence after six facets the discontinuity will be approximately10%. Thus, overcoming the problem of less intensity at thenon-overlapped beginning and end sections. The current configuration canbe repeated, in reverse, at the distal end of the waveguide. The slopeof change of reflectivity (dot-dashed and dashed lines) can be alteredto change the resulting effect and characteristic reflectivity of thesequence of facets. The waveguide can have a variety of combinations ofoverlapped and non-overlapped characteristic reflectivities over thelength of the waveguide. For example, the current figure facets “4” and“5” could be repeated over at least a portion of the sequence of facets,without converging to the configuration of facets “7” and “8”.

FIG. 20A illustrates a non-limiting but preferred process which may beused to produce first 1D waveguide 10. For clarity, in the drawings, theinternal facets are depicted not in scale or density.

A set of coated transparent parallel plates are attached together asstack 400. The stack is cut diagonally (402) in order to generate aslice 404. If required, a cover transparent plate 405 can be attached ontop and/or bottom (not depicted) of slice 404. The slice 404 is then cutperpendicular to the edges of the facets (dashed line on 404) if a 1Dfacet inclination is needed, or diagonally (dot-dashed line on 404) if a2D facet inclination is needed, to generate the 2D waveguide 406.

FIGS. 20B-20E are an exemplary procedure for attachment of a couplingprism. The sliced 2D waveguide 406 is shown in FIG. 20B with overlappingfacets (two facets reflecting per line of sight). This is a non-limitingexample only, and non-overlapping facets are also possible.

As illustrated in FIG. 20B, the 2D waveguide 406 (depicted nottransparent for clarity) is cut, for example, along the dotted line 420Aas illustrated. This cut can be at any orientation, but a perpendicularcut alleviates tight index matching requirements. Preferably, as seen inFIG. 20C, the cut is performed where the overlapping facets exist (seecut end in FIG. 20C) in order to maintain uniformity of illumination.Otherwise, the first facet will reflect without overlapping resultingwith reduced illumination. A transparent extension 413 can be added ifrequired and prism 414 is attached to the waveguide 406, generating a 2Dwaveguide 416 with an extension and coupling prism (as shown in FIG.20D). In cases where the extension is not needed, the coupling prism 414may be directly attached to the waveguide 406 to generate the assembledwaveguide 417 (as shown in FIG. 20D). The distal end of the waveguide406 may be left, to allow any remnant light to be scattered therefrom,and may optionally be painted with light absorbent material (e.g. blackpaint) to minimize stray reflections.

FIGS. 21A-21D are an exemplary procedure for creating a waveguide withoverlapping facets. In FIG. 21A, the sliced 2D waveguide 406 is cut, forexample, perpendicular along both sides, along the dotted line 420A atwhat will become the proximal end of the waveguide, and cut along thedotted line 420B at what will become the distal end of the waveguide.This produces waveguide 420 having partial facets 40 a and 40 b at therespective proximal and distal ends. In FIG. 21B, in this example,waveguides 426 and 424 are attached respectively to the proximal anddistal ends of the waveguide 420 and the combination (of 420, 424, and426) polished to produce in FIG. 21C smooth external surfaces (faces) ofcombined waveguide 428. In this combined waveguide 428, the attachedwaveguides 426 and 424 do not have to have as accurate a refractiveindex as waveguide 406.

This method of production can also be applied to waveguide withoutoverlapping facets, in order to eliminate the need for accuraterefractive index matching.

In FIG. 21D, optionally, the smoothness and optical properties of theexternal surfaces of the combined waveguide 428 can be improved byattaching external faces 427 having refractive index matching, toproduce waveguide 429.

Note, FIG. 21C and FIG. 21D depict separate components margins, however,for the light (such as incoming light ray 38), the margins aretransparent, and only the external faces and the slanted coated internalfacets reflect the light.

The current method (FIG. 20A to FIG. 21D) can be applied to a 1Dwaveguide as well as to a 2D waveguide.

The various embodiments in this description, such as varying facetspacing, width, and reflectivity, have been described separately forclarity. One skilled in the art will realize that these embodiments canbe combined. For example, varying facet spacing to be decreasing whilevarying the widths of the facets from the proximal to distal ends of thewaveguide.

Note that the above-described examples, numbers used, and exemplarycalculations are to assist in the description of this embodiment.Inadvertent typographical errors, mathematical errors, and/or the use ofsimplified calculations do not detract from the utility and basicadvantages of the invention.

To the extent that the appended claims have been drafted withoutmultiple dependencies, this has been done only to accommodate formalrequirements in jurisdictions that do not allow such multipledependencies. Note that all possible combinations of features that wouldbe implied by rendering the claims multiply dependent are explicitlyenvisaged and should be considered part of the invention.

It will be appreciated that the above descriptions are intended only toserve as examples, and that many other embodiments are possible withinthe scope of the present invention as defined in the appended claims.

What is claimed is:
 1. An optical device comprising: (a) a waveguide having: (i) a first of at least one pair of surfaces parallel to each other; (ii) a first region at which light is coupled into said waveguide; and (iii) a first sequence of facets including: (A) a first facet: (I) located proximally to said first region; and (II) having a first width in a direction between said first pair of surfaces; (B) a last facet: (I) at a distal end of said first sequence of facets from said first region; and (II) having a third width in a direction between said first pair of surfaces; and (C) one or more middle facets: (I) between said first facet and said last facet; and (II) having a second width in a direction between said first pair of surfaces; (b) wherein each of said facets: (i) width is in a plane of said facet; (ii) is an at least partially reflecting surface; (iii) is at an oblique angle to said first pair of surfaces; (iv) has a facet-start on a proximal side of said facet width; and (v) has a facet-end on a distal side of said facet width; and (c) wherein a geometrical projection is onto one of said first pair of surfaces in a direction of a nominal ray outcoupled from said waveguide, said nominal ray being a central ray of the light being coupled out of said waveguide, (d) said geometrical projection of said last facet and each of said one or more middle facets overlaps a respective said geometrical projection of an adjacent said one or more middle facets and said first facet, and (e) said geometrical projection of said facet-start of said last facet and each of said one or more middle facets coinciding with a respective said geometrical projection of a non-adjacent facet-end of said one or more middle facets and said first facet, (f) said coinciding along at least a portion of said waveguide.
 2. The optical device of claim 1 wherein said first width of said first facet is less than said second width of said one or more middle facets.
 3. The optical device of claim 1 wherein a number of said facets is crossed by said nominal ray outcoupled from said waveguide, said number of facets being constant for all of said first sequence of facets.
 4. The optical device of claim 1 wherein the light corresponds to an image and said central ray is a center ray from a center of said image.
 5. The optical device of claim 1 wherein the light corresponds to an image and said central ray corresponds to a central pixel of said image.
 6. The optical device of claim 1 wherein said last facet has a reflectivity that is substantially 100% of a nominal reflectivity, said nominal reflectivity being the total reflection needed at a specific location in said waveguide.
 7. The optical device of claim 1 wherein said third width is less than said second width.
 8. The optical device of claim 7 wherein said third width is substantially half of said second width.
 9. The optical device of claim 1 wherein a number of said one or more middle facets is selected from the group consisting of: (a) one; (b) two; (c) three; (d) four; and (e) five.
 10. The optical device of claim 1 wherein a constant number of facets overlap in a line of sight toward a nominal point of observation of said light coupling out of said waveguide via one of said first pair of surfaces.
 11. The optical device of claim 1 wherein a width of one of said facets of said first sequence of facets varies monotonically relative to a width of an adjacent one of said facets of said first sequence of facets.
 12. The optical device of claim 1 wherein a spacing between one pair of adjacent facets of said first sequence of facets varies relative to an adjacent spacing between another pair of adjacent facets of said first sequence of facets.
 13. The optical device of claim 12 wherein said spacing varies monotonically.
 14. The optical device of claim 1 wherein propagation of the light from said first region is such that at least a portion of the light encounters said first facet before encountering one of said one or more middle facets.
 15. The optical device of claim 1 wherein a spacing between adjacent facets is larger than the coherence length of the light being coupled into said waveguide.
 16. The optical device of claim 1 wherein (a) said first width is substantially equal to said second width; and (b) said first facet having a first section corresponding to said geometrical projection of said first facet that is nonoverlapping with said geometrical projection of an adjacent middle facet.
 17. The optical device of claim 16 wherein said first section is transparent to the light.
 18. The optical device of claim 16 wherein said first section has a reflectivity substantially twice a reflectivity of an adjacent facet.
 19. The optical device of claim 1 wherein each of said facets has uniform partial reflectivity across said facet.
 20. The optical device of claim 1 wherein a reflectivity of one or more of said facets varies from another reflectivity of another one or more facets.
 21. The optical device of claim 1 wherein said waveguide further has: (a) a second pair of surfaces parallel to each other and non-parallel to said first pair of surfaces; and (b) said facets configured such that, when an image is coupled into said waveguide at said first region with an initial direction of propagation at a coupling angle oblique to both said first and second pairs of surfaces, the image advances by four-fold internal reflection along said waveguide.
 22. The optical device of claim 21 wherein said second pair of surfaces are perpendicular to said first pair of surfaces.
 23. The optical device of claim 21 wherein each of said facets is at an oblique angle to said second pair of surfaces.
 24. The optical device of claim 1 wherein said waveguide further comprises: one or more external faces, each said external face attached to a surface of said first pair of surfaces, said external faces having refractive index matching to said first pair of surfaces. 